Passivation and oxidation of an ammonia iron catalyst

A process of ammonia iron catalyst passivation and oxidation with oxygen and water vapour was investigated by thermogravimetry, Mossbauer spectroscopy and X-ray diffraction. The optimal passivation conditions (T <= 100 degrees C, po(2) < 0.0013 bar) have been proposed. The thickness (0.91-1.1 nm) and the structure (paramagnetic Fe2O3) of a protective, passive layer have been determined. It was found that oxidation of a reduced catalyst with pure oxygen (po(2) = 1 bar) and water vapour (p(H2O) = 0.023 bar) did not lead to its passivation. The oxidation with pure oxygen, at temperatures above 250 degrees C, led to a creation of magnetite layer with built-in aluminum oxide. The thickness of the magnetite layer decreased and the amount of the built-in aluminum oxide grew along with the increase of oxidation temperature. An adsorption model has also been introduced for the interpretation of oxidation with water vapour: the whole iron nanocrystallites were oxidized successively from the smallest to the largest ones, and the oxidation process was limited by dissociative adsorption of water vapour on the iron surface. (c) 2007 Elsevier B.V. All rights reserved.